Power tools, such as circular saws, typically have a handle molded into the body of the tool. Such a handle is grasped by the power tool operator to guide and propel the tool through the workpiece. Usually, in a circular saw there is a rear handle and a forward handle. The rear handle oftentimes resembles a pistol-type grip. The handle extends upwardly and forwardly and is separated from the body of the saw so that the operator can easily grasp an elongated handle section that fits easily within the hand of the operator. This handle section typically extends in a direction that is generally parallel to and along the line of travel of the saw. As is apparent, it is extremely desirable to have the on/off switch for the saw located so that it can be actuated by at least the index and middle fingers of the operator's hand engaging the handle. Such an arrangement allows an operator to selectively start and stop the cutting operation of the saw while having his/her hand gripping the handle.
Many prior power tool constructions have a lockout mechanism also associated with the handle structure which holds the switch on the handle in a locked position and requires the operator to actuate the mechanism prior to turning the power tool to the "on" position utilizing the switch. In particular, many of these prior structures require an operator to actuate a separate button or lever with his/her thumb prior to or simultaneously with actuation of the switch by the index and middle finger of the operator's hand gripping the handle.
Prior lockout mechanisms or latches typically are of two main types, a pivoting type and a sliding type. In a pivot type arrangement, the latch is pivotally mounted within the handle structure about an axis which is transverse or perpendicular to the elongated direction of the handle. In the case of a circular saw, the latch is pivotally mounted about an axis that is parallel to the axis of rotation of the saw blade. These latches operate by pivoting between an engaged position wherein the handle switch contacts the latch member and is prevented from movement to its "on" position, and a disengaged position wherein the operator is allowed to actuate the switch to the "on" position. Examples of these transverse pivotal lockout mechanisms can be found in U.S. Pat. Nos. 3,873,796 and 5,577,600. In each of these references, the latch mechanism is actuated by a button located on the top surface of the handle. In particular, they require either the pushing of the button or the rotating of the button rearwardly to allow actuation of the switch. These structures are disadvantageous for various reasons. In particular, the location of the lockout mechanism button on the top surface of the handle requires the positioning of the thumb in an awkward position. More specifically, it is natural when gripping a handle for the thumb to be along the side of the handle with the cross section of the handle received between the thumb and index finger. As is apparent, to actuate the mechanisms in these references, the thumb must first be positioned on the top of the handle, thus resulting in a less secure grip on the handle. Such loose gripping can result in misalignment of the saw during its initial cutting actions. Still further, in these prior references, for the thumb to reach the normal gripping position on the side of the handle, the thumb must slide off the button and over the side of the handle. The friction associated with the thumb passing over the top of the handle and the awkward sideward movement of the thumb can result in operator discomfort during the initial cutting action of the saw.
A still further disadvantage of these references is the location of the lockout mechanism actuating button above or behind the on/off switch with respect to the longitudinal axis of the handle. More specifically, when a person typically grabs a handle, the tendency is for the thumb to be forward of the index and middle fingers. To actuate the lockout mechanism buttons of these references, the thumb must be moved rearwardly to push the actuating button, thus presenting a potential awkward position for the saw operator, and, further, possibly resulting in unnecessary reorientation of the thumb along the side of the handle to the normal gripping position.
The second type of lockout mechanism includes a latch member which, when actuated, slides within the handle housing to allow actuation of the on/off switch by the operator. An example of this type of sliding latch member is disclosed in U.S. Pat. No. 5,638,945. These sliding lockout mechanisms are oftentimes relatively complicated and do not allow ergonomic positioning of the thumb during the beginning power tool operation. More specifically, the structure of the above reference, again, has the actuating switch positioned on the top surface of a handle housing and at a location that is above the actuating switch for the power tool. Thus, an operator, to operate the power tool, is required to position his or her thumb on the top of the handle instead of along the side, and to push the lockout mechanism button forward on the upper surface while pushing upward on the switch, and thereafter to slide the thumb of the hand positioned on the handle to the side of the handle to the normal comfortable gripping position. As with the pivoting latch mechanisms discussed above, this sliding-type mechanism is highly disadvantageous because it requires the operator to utilize significant effort to reposition his or her thumb in a normal gripping operation, and also has the sliding actuating switch or button located directly above the on/off switch which is typically not a normal position for a hand gripping the handle.
A still further disadvantage of all the above lockout mechanisms is the structure used to bias the lockout mechanism back to its original locked position. In particular, the prior mechanisms tend to utilize leaf springs or deformable arms to supply the biasing force. These types of biasing structures are disadvantageous because the spring force of the structure increases generally from zero along a generally linear type path with further deformation of the spring or arm. In other words, as these springs become more deformed, they offer more resistance. As is apparent, this is disadvantageous to an operator because his/her thumb must increase force with further actuation of the lockout button or lever, thus again causing more uncertainty, and less stability during initial cutting operations. Some prior art structures also utilize coil springs compressed along their central axis. These coil springs compressed in this way also have a generally linear spring force curve and are disadvantageous for the same reasons as the other biasing structures.
Therefore, a lockout mechanism is needed which will overcome the problems with the prior art lockout mechanisms discussed above.